Electronics Guide

Intermodulation Product Frequencies

For two input signals at frequencies f1 and f2, intermodulation products appear at frequencies:

f_IM = m*f1 + n*f2

where m and n are integers (positive, negative, or zero). The order of the product is |m| + |n|. The most important products for PIM testing are typically:

• Third-order products (IM3): 2f1 - f2 and 2f2 - f1 (order 3)
• Fifth-order products (IM5): 3f1 - 2f2 and 3f2 - 2f1 (order 5)
• Seventh-order products (IM7): 4f1 - 3f2 and 4f2 - 3f1 (order 7)

In wireless systems, the test frequencies are typically chosen in the transmit band such that the lower third-order product (2f1 - f2) falls in the receive band. This represents the worst-case scenario where PIM directly interferes with reception.

For example, in an LTE Band 7 system with transmit frequencies around 2620-2690 MHz and receive frequencies around 2500-2570 MHz, test frequencies might be f1 = 2617 MHz and f2 = 2680 MHz, placing the lower IM3 product at 2554 MHz in the receive band.

Power Level Relationships

For an ideal polynomial nonlinearity, PIM power follows a predictable relationship with carrier power:

P_IM3 increases by 3 dB for every 1 dB increase in carrier power

This 3:1 slope (in dB) applies to third-order products from a purely third-order nonlinearity. Similarly, fifth-order products increase at 5:1, and seventh-order at 7:1.

In practice, real PIM sources often deviate from these ideal slopes due to:

• Multiple nonlinear mechanisms with different order dependencies
• Thermal effects that change with power level
• Saturation of contact or material nonlinearities
• Distributed PIM sources with frequency-dependent coupling

Measuring PIM at multiple power levels and comparing to the theoretical slope provides diagnostic information about the PIM source type.

Standard Test Conditions

Industry standards specify test conditions for component qualification:

Power level: The standard test power is typically 2 x 20 watts (2 x 43 dBm) per carrier. Some applications require higher power (2 x 40 watts or more). The power should be specified at the DUT input, accounting for any cable and adapter losses.

Frequency selection: Test frequencies should be chosen to place intermodulation products in bands where they would cause interference. For cellular systems, this typically means IM3 products in the receive band.

Measurement bandwidth: A narrow receiver bandwidth (typically 100 Hz to 10 kHz) improves sensitivity but increases measurement time. The bandwidth should be wide enough to capture any frequency instability in the PIM product.

Test duration: PIM can vary over time due to thermal effects, and measurements should be made at thermal equilibrium. Standard test procedures specify minimum stabilization times.

Frequency Sweep Methods

Several approaches are used for swept PIM testing:

Single-carrier sweep with fixed offset: One carrier is swept while maintaining a fixed frequency offset between the two carriers. The IM3 product sweeps across a corresponding frequency range. This method reveals the frequency dependence of both PIM generation and system response.

Dual-carrier sweep with tracking: Both carriers are swept such that the IM3 product remains at a fixed frequency. This isolates the frequency dependence of PIM generation from receiver response variations.

Stepped frequency: Multiple discrete frequency pairs are measured sequentially. This is simpler to implement but provides less frequency resolution than continuous sweeping.

The sweep rate must be slow enough to allow accurate measurement at each frequency. Thermal transients and measurement settling time limit how fast the sweep can proceed.

Interpreting Swept PIM Data

Swept PIM measurements reveal features that fixed-frequency tests miss:

• Resonances: Sharp peaks in PIM versus frequency indicate resonant structures that enhance PIM at specific frequencies
• Cable length signatures: Periodic variations with frequency can indicate PIM from specific cable lengths or component locations
• Multiple sources: The frequency dependence may reveal contributions from multiple PIM sources with different characteristics
• Receiver response: When only one carrier is swept, receiver sensitivity variations contribute to the measured frequency response

Swept testing is particularly valuable for system-level evaluation where multiple components contribute to overall PIM, and for troubleshooting to identify which components or locations are responsible for elevated PIM.

Speed and Resolution Trade-offs

Swept PIM testing involves trade-offs between speed, frequency resolution, and measurement accuracy:

Faster sweeps: Reduce test time but may miss narrow PIM resonances and provide less accurate measurements at each point

Higher resolution: More frequency points provide finer detail but increase test time proportionally

Averaging: Multiple measurements at each frequency improve accuracy but multiply test time

For production testing, the sweep parameters are typically optimized to detect PIM problems quickly while maintaining adequate measurement confidence. For diagnostic testing, slower sweeps with more detail may be warranted.

Modulation Effects on PIM

Modulated signals differ from CW in ways that affect PIM measurement:

Power variation: Modulated signals have varying instantaneous power. High peak-to-average power ratio (PAPR) signals like OFDM can produce peak powers well above the average, potentially exciting PIM at levels higher than CW testing at the same average power.

Spectral distribution: The broadband spectrum of modulated signals creates a continuum of intermodulation products rather than discrete tones. This distributes PIM energy across a frequency band.

Temporal variation: The time-varying envelope of modulated signals means PIM levels fluctuate rapidly. Thermal PIM mechanisms may respond differently to modulated versus CW excitation.

For system-level evaluation, testing with representative modulated signals provides the most realistic assessment of PIM impact.

Noise Power Ratio Testing

Noise power ratio (NPR) is a modulated-signal test method that characterizes nonlinearity across a continuous frequency band:

A broadband noise signal with a notch (gap) at one frequency is applied to the DUT. Nonlinearity fills in the notch with intermodulation products. The ratio of out-of-notch power to in-notch power is the NPR.

NPR testing captures the aggregate effect of all orders of intermodulation and is sensitive to the same nonlinearities that cause PIM. However, it does not directly measure PIM at specific frequencies and may be dominated by contributions from active components rather than passive nonlinearities.

Correlation with CW Results

For standardization and comparison purposes, CW two-tone testing remains the standard method. Modulated signal tests complement rather than replace CW testing:

Component qualification: CW testing at specified power levels provides reproducible, comparable results for vendor specifications and acceptance testing

System verification: Modulated signal testing verifies that CW-qualified components perform adequately under realistic operating conditions

Correlation: The relationship between CW and modulated PIM depends on the modulation format, but CW testing at appropriate power levels typically provides a conservative estimate of modulated-signal PIM

Frequency-Domain Distance Measurement

The basic principle uses the phase rotation of reflected signals to determine distance:

When the test frequencies are swept, PIM products from a single source show a phase that varies linearly with frequency. The rate of phase change is proportional to the electrical distance to the PIM source:

Distance = (c * delta_phi) / (4 * pi * delta_f * sqrt(epsilon_r))

where delta_phi is the phase change, delta_f is the frequency change, c is the speed of light, and epsilon_r is the relative permittivity of the transmission line.

Advanced signal processing (such as inverse Fourier transforms) converts the frequency-domain phase information to a time-domain impulse response, showing the distance to each PIM source as a distinct peak.

Interpreting Distance-to-PIM Results

DTP measurements show the location of PIM sources along the RF path:

• Single peak: A dominant PIM source at the indicated distance (connector, cable fault, or other discrete source)
• Multiple peaks: Several PIM sources at different locations, which may all need attention
• Broad response: Distributed PIM (from a cable or antenna) rather than a discrete source
• Near-zero distance: PIM in the test equipment, adapters, or very close to the test port

Distance accuracy depends on knowledge of the cable velocity factor, the frequency sweep range (wider sweeps give better resolution), and the signal-to-noise ratio of the PIM measurement.

Practical DTP Considerations

Effective use of DTP requires attention to several factors:

Reference plane: The measurement reference point (zero distance) is at the test port. Adapters, test cables, and loads all appear at their physical distances from this point.

Velocity factor: Accurate distance calculation requires knowing the velocity factor of each cable segment. Mixed cable types create multiple velocity factors that complicate interpretation.

Resolution: The distance resolution is inversely proportional to the frequency sweep bandwidth. A 100 MHz sweep provides approximately 1.5 meter resolution in cable; wider sweeps provide finer resolution.

Multiple sources: When multiple PIM sources are present, they can interfere constructively or destructively at different frequencies, complicating the distance calculation. Advanced algorithms attempt to separate overlapping sources.

Test System Architecture

A typical PIM test system includes:

Signal generators: Two synthesized signal sources provide the test carriers. Frequency accuracy and phase noise are important for precise IM3 frequency targeting and narrow-band measurement.

Power amplifiers: High-power linear amplifiers boost the test signals to the required level (typically +43 to +46 dBm). These amplifiers must have extremely low distortion to avoid generating PIM within the test equipment.

Combining network: A diplexer or hybrid combiner combines the two carriers for application to the DUT. The combiner must have high isolation between ports and low PIM.

Duplexer/filter: A sharp filter separates the receive path from the transmit path, attenuating the carriers by 80-100 dB in the receive path while passing the IM3 frequency.

Low-noise receiver: A sensitive receiver with low noise figure measures the PIM products. Dynamic range and linearity are critical to avoid receiver-generated intermodulation.

Control and processing: A controller coordinates the measurement, performs calibration, and processes results.

Portable Field Testers

Field-portable PIM analyzers integrate all functions into a single instrument suitable for tower climbing and site work:

• Battery operation: Allow testing at locations without AC power
• Rugged construction: Withstand the rigors of outdoor use and transport
• Integrated display: Provide immediate pass/fail indication and graphical results
• Simplified operation: Pre-programmed test profiles for common band and protocol combinations
• DTP capability: Many modern portable testers include distance-to-PIM functionality

Portable testers typically offer power levels up to 2 x 40 watts and sensitivity adequate for testing to specification limits. For more demanding requirements, laboratory systems provide higher power and better sensitivity.

Laboratory Test Systems

Laboratory PIM test systems are designed for component qualification and research applications:

• Higher power: Power levels up to 2 x 100 watts or more for stress testing
• Better sensitivity: Noise floors below -175 dBm with narrow bandwidths
• Environmental chambers: Integration with temperature and humidity control for environmental testing
• Automation: Support for automated test sequences and data logging
• Multiple bands: Coverage of multiple frequency bands with interchangeable front-end assemblies

Laboratory systems typically use rack-mounted modular components, allowing configuration for specific test requirements and future upgrades.

Carrier-to-PIM Ratio

Consider a typical scenario: two +43 dBm carriers (20 watts each) generate a PIM product at -110 dBm (typical specification limit for low-PIM components). The ratio between carrier power and PIM power is:

43 dBm - (-110 dBm) = 153 dB

To measure 10 dB below this specification limit requires 163 dB of dynamic range. Measuring 20 dB of margin requires 173 dB. These numbers illustrate why PIM measurement is among the most demanding RF measurements.

Achieving Adequate Isolation

The test system must prevent the powerful carriers from reaching the receiver, where they would either overload the receiver or generate intermodulation within the receiver itself. This is accomplished through:

Diplexer/duplexer filtering: Sharp bandpass and bandstop filters in the receive path attenuate the carriers by 80-100 dB while passing the IM3 frequency with minimal loss.

Directional coupling: Using directional couplers to separate forward and reflected signals provides additional isolation.

Physical separation: Isolating transmit and receive electronics prevents coupling through housings, cables, and circuit board traces.

Shielding: High-quality shielding prevents RF leakage between transmit and receive sections.

Even with all these measures, some carrier energy reaches the receiver. The receiver must be linear enough to handle this residual carrier without generating false PIM.

Residual PIM and System Floor

Every PIM test system has a residual PIM level, the PIM generated by the test system itself when connected to a perfect (PIM-free) load. This residual sets the measurement floor and limits the minimum PIM that can be measured.

Residual PIM arises from:

• Connectors, adapters, and cables in the test setup
• The combiner and diplexer networks
• Amplifier distortion products
• Receiver intermodulation

High-quality test systems achieve residual PIM below -165 dBc (relative to total carrier power) or -125 dBm absolute at 2 x 43 dBm. This allows accurate measurement of components specified at -110 dBm with margin for production variations.

Verifying residual PIM requires a low-PIM termination at the test port. The measured PIM with this load represents the system floor. Measurements of DUTs are valid only when significantly above this floor.

Power Level Uncertainty

The carrier power at the DUT affects PIM level according to the 3:1 (dB) relationship. Uncertainty in carrier power translates to uncertainty in expected PIM:

1 dB carrier power uncertainty causes 3 dB PIM uncertainty

Power accuracy depends on the calibration of power meters or directional couplers, cable losses, and connector losses. Regular calibration and careful loss compensation are essential.

Receiver Calibration

The accuracy of PIM level readings depends on receiver calibration:

• Absolute amplitude accuracy: How accurately does the receiver report signal level?
• Linearity: Does the receiver maintain accuracy across its measurement range?
• Frequency response: Does sensitivity vary across the measurement frequency range?
• Noise figure: Does thermal noise affect measurement of low-level PIM?

Calibration using traceable signal sources establishes receiver accuracy. The calibration should cover the full frequency and amplitude range used in testing.

Repeatability

PIM measurements often show significant variation between repeated measurements, even with no changes to the setup. Causes include:

Connector variation: Each time a connector is mated, the contact conditions differ slightly

Thermal drift: Temperature changes in the test system and DUT cause PIM variations

Mechanical settling: Vibration and movement cause PIM fluctuations

Time-varying PIM: The DUT itself may exhibit temporal PIM variations

Good practice includes multiple measurements with averaging, consistent torquing of connections, and adequate thermal stabilization before measurement.

Traceability and Standards

PIM measurements are difficult to trace to fundamental standards because the measurand (intermodulation in passive devices) is not independently realizable. Traceability is established through:

Transfer standards: Stable, characterized DUTs with known PIM levels serve as reference standards. These are periodically verified against other references through round-robin testing.

Synthetic PIM sources: Some systems generate artificial PIM using a controlled nonlinear device (such as a diode). This verifies receiver and system response but does not replicate all characteristics of real PIM.

Calibration services: National metrology laboratories and commercial calibration labs provide calibrated reference devices and measurement validation services.

System-Level Testing

Field PIM testing typically measures the complete path from the test port to the antenna and back. This includes:

• Main feeder cables
• Jumper cables
• Connectors (multiple)
• Lightning protection devices
• Antenna feed systems
• Antenna elements
• Any mounting hardware, tower members, or other metallic objects in the antenna's near field

The measured PIM is the aggregate of all sources in this path. When PIM exceeds specification, distance-to-PIM testing helps identify which component is responsible.

Environmental Considerations

Outdoor installations experience environmental conditions that affect both PIM performance and measurement:

Temperature: PIM often varies significantly with temperature. Field tests should note ambient temperature and, when possible, test at temperature extremes.

Wind: High winds can cause cable and antenna movement, creating vibration-induced PIM that may not be present in calm conditions.

Moisture: Water ingress into connectors or cables degrades PIM performance. Testing after rain may reveal moisture-related problems.

Solar heating: Sun exposure can cause thermal gradients and elevated temperatures that affect PIM.

Weather conditions should be documented with field test results to aid interpretation and comparison.

Safety Considerations

Field PIM testing involves high-power RF that presents safety hazards:

RF exposure: Power levels used for PIM testing can exceed safe exposure limits. The antenna should be radiating away from occupied areas, or barriers should prevent access to high-field regions.

Interference: Test signals can interfere with operating wireless systems. Coordination with network operators is required, and testing may need to occur during maintenance windows.

Equipment hazards: Working on towers and with heavy test equipment requires appropriate training and safety procedures.

Test procedures should include safety protocols, including RF lockout/tagout, exposure monitoring, and coordination with affected parties.

Documentation and Records

Field test documentation should include:

• Site identification and test location
• Date, time, and weather conditions
• Equipment used (model and serial numbers)
• Test frequencies and power levels
• Measured PIM levels and pass/fail status
• Distance-to-PIM results if obtained
• Any remedial actions taken
• Post-remediation test results

This documentation provides a baseline for future comparison and evidence of installation quality for acceptance purposes.

Low-PIM Adapters and Cables

Adapters and cables connecting the test instrument to the DUT must have PIM well below the measurement requirements:

Adapter PIM: Low-PIM adapters are specified at -160 dBc or better. Standard commercial adapters may have PIM as high as -120 dBc, unsuitable for most testing.

Cable assemblies: Test cables should use solid or corrugated outer conductors and high-quality connectors. Braided cables can contribute significant PIM.

Connector torque: All threaded connections should be tightened to specification using a torque wrench. Under-torqued connectors are a common cause of elevated measurement floor.

Test accessories should be verified periodically by measuring with a known low-PIM load. Degradation over time from wear or damage requires replacement.

Low-PIM Terminations

A low-PIM termination (load) is essential for system verification and serves as a reference for DUT measurements:

• Must handle the full test power (typically 40-80 watts or more)
• Should have PIM below the system residual level
• Requires high-quality connector matched to test port type
• May need cooling for continuous high-power operation

Low-PIM loads use precision resistive elements with non-ferromagnetic materials throughout. The load connector is often the limiting factor in PIM performance.

Calibration Standards

Calibration of PIM test systems requires specialized standards:

PIM reference devices: Components with stable, known PIM levels serve as check standards. These may be manufactured devices or carefully characterized samples maintained as references.

Synthetic PIM sources: Electronic devices that generate controlled intermodulation provide repeatable signals for receiver calibration and system verification.

Through connections: Low-PIM through adapters allow testing the complete signal path without a DUT, verifying system residual.

Setup and Preparation

• Inspect all connectors for damage, contamination, or wear before each use
• Clean connector interfaces with appropriate solvents and lint-free wipes
• Verify connector gauge dimensions if available
• Use a torque wrench to achieve specified connector torque
• Allow equipment to warm up and stabilize thermally
• Verify system residual PIM with a low-PIM load before testing DUTs

During Measurement

• Avoid touching or moving cables during measurement
• Shield the setup from air currents that cause thermal fluctuations
• Monitor for unstable readings that indicate intermittent PIM sources
• Record ambient temperature and note any environmental changes
• Use appropriate averaging to reduce measurement noise
• Verify results by repeating measurements

Troubleshooting Elevated Readings

When PIM exceeds expectations, systematic troubleshooting identifies the source:

1. Verify system residual with low-PIM load (problem in test system?)
2. Replace test cables and adapters (problem in accessories?)
3. Re-mate connectors with proper torque (connection problem?)
4. Use distance-to-PIM to locate the source
5. Substitute components to isolate the problem
6. Inspect suspect components for damage or contamination

Many apparent DUT failures prove to be test system issues upon investigation.